Electric hygrometer



April 1, 1941. KQLLER 2237300 ELECTRIC HYGROMETER Filed April 21, 1958ALUMINUM Imvefitor. Lewis R.Koller;

t-hs Attorney.

'such as a metallic oxide I2.

Patented Apr. 1, 1941 ELECTRIC HYGROMETER Lewis R. Keller, Schenectady,N. Y., assignor to General Electric Company, a corporation of New YorkApplication April 21, 1938, Serial No. 203,297

Claims.

My invention relates to humidity or moisture indicators and regulatorsand concerns particularly humidity responsive elements which vary inelectrical conductivity with variations in humidity,

It is an object of my invention to provide a humidity responsive elementhaving a high conductivity and which will carry sufficient current tooperate contact-making and indicating devices and instruments directly.

It is a further object of my invention to provide a humidity responsiveelement which responds quickly to variations in humidity and quicklyreaches equilibrium after a change in humidity. Other and furtherobjects and advantages will become apparent as the description proceeds.

In carrying out. my invention in its preferred form I provide a base ofelectrically conducting material on the surface of which is coated avery thin film of hygroscopic material such as a metallic oxide, whichis covered with a foil of material which can be beaten very thin, suchas gold foil. A voltage is applied between the two surfaces of the oxidefilm by means of terminals connected to the metallic base and to thefoil covering, and variations in current are observed to determinevariations in humidity- The invention may be understood more readilyfrom the following detailed description when considered in connectionwith the accompanying drawing and those features of the invention whichare believed to be novel and patentable will be pointed out in theclaims appended hereto.. In the drawing Figure 1 is a perspective viewdiagrammatically representing one embodiment of my invention. Figure 2is a fragmentary enlarged cross sectional view of the arrangement ofFigure 1. Figure 3 is a circuit diagram of a humidity indicatorutilizinga humidity responsive element of the type illustrated in Figures 1 and2; and Figure 4 is a circuit diagram of a humidity controllercorresponding to the arrangement of Figure 3.

Like reference characters are utilized to designate like partsthroughout the drawing.

The humidity responsive element illustrated in Figure 1 comprises aplate ll of conducting material serving as a base, the surface of whichis coated with a suitable hygrometric material surface I 2 of the baseII a cover of very thin conducting material such as metallic foil I3 islaid and electrical connections are provided for the base H and the foilcover l3. If desired. the foil l3 may be electrically and mechanicallyrein- Upon the coated forced for the electrical connection by providinga sheet I of thin electrical conducting material 'in contact with aportion of the surface of the numeral I! in Figures 3 and 4 and theelement I1 is connected to a source of alternating current IS in serieswith a current responsive device such as an indicating instrument ill orcontact-making circuit controlling device or a relay 20. In Figure 4 thecontact-making device 20 is shown with a movable contact 2! and a pairof stationary contacts 22 included in an electrical circuit 23 which maybe an operating circuit for any suit able type of humidity varyingdevice 24 such as, for example, a blower for introducing humidifled airinto a chamber or a steam valve for controlling the heating of a chamberto dry the atmosphere, or the like.

Although my invention is not limited to the use oi. particular chemicalsubstances for constructing the humidity responsive element H, I havefound that satisfactory results may be obtained by utilizing as the basei I a plate of aluminum and forming a film of aluminum oxide l2.

thereon electrolytically'in a sulphuric acid electrolyte. Although notlimited to the specific thinness of film I have found a thickness ofapproximately of a mil to be satisfactory. I have obtained satisfactoryresults by using gold leaf as the cover I3 for the aluminum oxide and Ibelieve that golf leaf has the advantage of permitting moisture to passthrough it quickly to the aluminum oxide or to the atmosphere. In viewof this property of the foil and the extreme thinness with whichaluminum oxide can be prepared I find that the humidity responsiveelement reaches equilibrium very quickly after a change in the humidityof the surrounding atmosphere, so that my humidity responsive element isuseful in cases where quick response to variations in humidity isdesired.

I have found that the conductivity of the aluminum oxide film l2 variesapproximately linearly with the aqueous vapor tension of the surroundingatmosphere for the humidities in the range usually experienced inordinary rooms.

With a -volt alternating current source I have found the relationshipbetween aqueous vapor tension of the atmosphere and current inmicroamperes for every square centimeter of surface of oxide film to beapproximately as represented in the following table:

Microamperes per sq. cm. at

Aqueous vapor tension (in mm. of Hg.)

100 volts It will be obvious that the current-carrying capacity of thedevice may be varied by varying the surface area of the film.

The electrical supply source l8 may be either alternating current ordirect current but where the apparatus is to be in operation forextended periods of time I prefer to utilize an alternating currentsource since I find that there is no fatigue effect with alternatingcurrent.

Although I have obtained satisfactory results by the use of an aluminumbase Ii it will be understood that my invention is not limited thereto.For example, I may employ a base of chromium metal with the surfaceoxidized. Likewise, I have found that satisfactory results are obtainedfrom a steel base on which chromium oxide has been painted to form thehygroscopic film. The hygroscopic film may also be formed by dipping asteel plate into a mixture of chrome oxide, acetone, and a small amountof alkyd resin or varnish such as that sold under the trade nameGlyptal." It will be observed that both aluminum oxide and chromiumoxide are refractory oxides.

The foil cover of the arrangement illustrated in Figure 2 consists of asheet of commercial gold leaf, which has a thickness of approximately10- centimeters, and is applied to the oxide film I2 by merely layingthe foil on the coated base i I, connecting the terminals [5 and it to avoltage source and allowing the electrostatic attraction to hold thefoil against the coated plate. After a short time, I find that the foill3 adheres firmly even without the electrostatic attraction. Although Ihave found such a golf leaf cover satisfactory, my invention is notlimited to the use of this precise form of cover. For example, thethickness, the

material, or the method of producing may be leai upon the oxide film l2,the cover l3 may, for

example, be applied by the evaporating process or by cathode sputteringin a manner somewhat similar to these well known processes of producingthin metallic coatings for use as mirror surfaces. In the specificationand claims, I use the word foil to specify any very thin sheetregardless of the method by which it is produced.

Aluminum oxide has a volume specific resistivity of approximatelymegohmn cm. I am enabled to use such a high resistance hygroscopicmaterial effectively by utilizing a very thin film of large area andpassing the current through it transversely. I believe that the aluminumoxide formed in a film as described acts as a-spongy holder foratmospheric water which is hydrolized by the electric field and carriescurrent varying with the humidity.

I have herein shown and particularly described certain embodiments of myinvention and certain methods of operation embraced therein for thepurpose of explaining its principle and showing its application but itwill be obvious to those skilled in the art that many modifications andvariations are possible and I aim, therefore, to cover all suchmodifications and variations as fall within the scope of my inventionwhich is defined in the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A humidity responsive element which varies in electrical conductivitywith variations in humidity, said element comprising an aluminum basehaving an electrolytic refractory oxide film thereon and a gold foillying against and substantially covering the oxide film, with electricterminals connected to the aluminum base and to the gold foil, said filmcomprising a moisture sensitive oxide of the group consisting ofaluminum oxide and chromium oxide, and said gold foil being sufficientlythin to permit atmospheric moisture to reach and affect the oxide film.

2. A humidity responsive element which varies in electrical conductivity.with variations in humidity, said element comprising a base ofconducting material having a film of aluminum oxide thereon of athickness less than one mil and a gold foil lying against the oxidefilm, with electric terminals connected to the base and to the goldfoil.

3. A humidity responsive element which varies in electrical conductivitywith variations in humidity, said element comprising an aluminum basewith an oxidized surface and a thin metallic foil lying against theoxidized surface, with electric terminals connected to the aluminum baseand to the metallic foil. 7

4. A humidity responsive element which varies in electrical conductivitywith variations in humidity, said element comprising a base of anelectrical conducting material, a film composed of a refractory metallicoxide coated on the surface of the base and a thin metallic foil lyingagainst and substantially covering the oxide film with electricterminals connected to the base and to the foil, said film comprising amoisture sensitive oxide of the group consisting of aluminum oxide andchromium oxide, and said metallic foil being sufiiciently thin to permitatmospheric moisture to reach and effect the oxide film.

5. A humidity responsive element which varies in electrical conductivitywith variations in humidity, said element comprising a film of hygro-CERTIFICATE OF CORRECTION. Patent No. 2,257,006. April 1, 191a.

LEWIS R. KOLLERV It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionas follows: Page 1, second column, line 56, for the word "golf" read--gold; page 2, second col umn, line 55, claim 1;, for "effect" read--affect--; and that the said Letters Patent should be read with thiscorrection therein that the same may conform to the record of the casein the Patent Office.

Signed and sealed this 15th day of May, A. D. 19141.

- Henry Van Arsdale, (Seal) Acting Commissioner 0% Patents.

v CERTIFICATE OF CORRECTION. Patent No. 2,257,006. April 1, 19m.

' LEWIS R. KOLLER.-

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Pagel,second column, line 56, for the word "golf" read --gold-; page 2, secondcolumn, line 55, claiml for "effect" read --affect-; and that thesaid'Letters Patent should be read with this correction therein that thesame may conform to the record of the case in the Patent Office.

Signed and sealed this 15th day of May, A. D. wi l.

I Henry Van Arsdale, (Seal) Acting Commissioner 0% Patents.

